During cardiovascular surgery and cardiac valve surgery (commonly known as open heart surgery) it is often necessary to arrest the pumping activity of the heart by inducing heart paralysis or "cardioplegia." While the heart is arrested it is also necessary to replace or supplement the heart and lungs by means of a cardiopulmonary support circuit (commonly known as a heart-lung machine). The cardiopulmonary support circuit receives blood from a major vessel entering the heart, such as the vena cava. The blood received from the heart is typically transported to a venous reservoir and then pumped through an oxygenator by an arterial pump and transported back to the patient via an arterial line, to enter the patient through a major vessel leaving the heart, such as the aorta. The pumping action of the arterial pump temporarily replaces the action of the heart and the oxygenator temporarily replaces the action of the lungs. It is thereby possible to deliver oxygenated blood to the tissues of the patient during the open heart surgery while the patient's heart is stopped. The cardiopulmonary circuit typically also includes a cardioplegia circuit, a cardiotomy circuit and a ventricular vent circuit. The cardiopulmonary circuit is typically operated in the operation room by a medical professional known as a "perfusionist."
The cardioplegia circuit delivers a cardioplegia mixture to the heart. The cardioplegia mixture discontinues the beating of the heart in a manner that minimizes damage to the heart's myocardium and renders the heart motionless so that a surgeon may operate. Cardioplegia mixture may also supply other ingredients to provide for myocardial protection. Cardioplegia mixture may comprise a crystalloid solution delivered alone or may also include oxygenated blood diverted from the arterial line. The crystalloid solution typically contains potassium chloride (KCl), sugars and magnesium. The potassium cation (K.sup.+) concentration in the cardioplegia is initially elevated, for example 25 mmol/l during induction of arrests, and thereafter reduced, for example 8.5 to 9.0 mmol/l, during the remainder of the procedure. Other cations, such as magnesium (Mg.sup.++) can be used as heart arresting agents.
It is generally preferred to include oxygenated blood, diverted from an arterial portion of the cardiopulmonary support circuit, in the cardioplegia mixture, so that the cardioplegia mixture is buffered and can oxygenate the myocardium. Where the cardioplegia mixture includes diverted arterial blood the cardioplegia circuit typically comprises diverted blood tubing, for containing blood diverted from an arterial blood line, a crystalloid solution bag and tubing, a cardioplegia delivery tubing conveying the mixture of blood and crystalloid solution, and one or more pumps, which are typically peristaltic pumps.
The crystalloid solution tubing and the diverted blood tubing may both comprise pump headers which may both be threaded through the same peristaltic pump or through different peristaltic pumps. The pumps may be other pump types as are known in the art.
The cardioplegia mixture is generally cooled to render it hypothermic, further protecting the myocardium during open heart surgery. The cardioplegia mixture may be administered antegrade, directly into the coronary ostia of the heart, or retrograde, through the right atrium or the coronary sinus, depending upon the surgical need. The cardioplegia mixture is then distributed through the circulatory system of the patient in combination with the blood perfused into the aorta or, occasionally, it may be withdrawn from the chest cavity and discarded or directed via a cardiotomy line to the cardiopulmonary support circuit.
In one prior art system, a peristaltic cardioplegia pump is provided to pump diverted arterial blood from the arterial blood line for combination with crystalloid solution, which is pumped by a separate crystalloid pump. The diverted arterial blood pumped by the cardioplegia pump and the crystalloid solution pumped by the crystalloid pump are combined downstream of the pumps to create the cardioplegia mixture. By controlling the speeds, and hence the pumping rates, of the crystalloid pump and the cardioplegia pump the perfusionist can control a flow rate of crystalloid solution and the flow rate of diverted arterial blood, thereby controlling both the total flow of cardioplegia mixture and the concentration of crystalloid solution in the cardioplegia mixture, that is a ratio of crystalloid solution to diverted arterial blood. In this prior art system, it is possible to control the concentration of crystalloid in the cardioplegia mixture, but the dilution of the diverted arterial blood by the crystalloid solution depends on the proportion of crystalloid solution. That is, as the proportion of crystalloid solution is increased, the dilution of the diverted arterial blood is also increased. This is important because dilution of the diverted arterial blood decreases the proportion of red blood cells, the oxygen carrying component of the arterial blood, reducing the cardioplegia mixture's ability to deliver oxygen to the tissues of the myocardium. A further undesirable consequence of cardioplegia mixture administration is dilution of the total blood volume of the patient. It is, therefore, desirable to control and limit this dilution during surgery. This dilution can be controlled by use of a hemoconcentrator, as is well known, but such hemoconcentrators increase the cost and complexity of the cardiopulmonary circuit. Concentrations of other ingredients in the crystalloid solution vary with the potassium concentration in the mixture. It may be desirable to keep other concentrations fixed while varying only the potassium concentration.
Another prior art system employs a cardioplegia pump which is a peristaltic pump having two pump headers tube in the same raceway. One pump header tube carries the diverted arterial blood and the other carries the crystalloid solution from the source of crystalloid solution. The ratio of the two pump headers is selected to fix the dilution of the diverted arterial blood at a constant value. Changes in potassium concentration are achieved by switching bags of crystalloid solution. The heart is typically arrested with a high concentration solution, then the bag is switched to a lower concentration for maintenance of arrest. In this prior art system, the volume of cardioplegia mixture delivered to the heart is increased and decreased in order to increase and decrease the amount of crystalloid delivered to the heart.
It is desired to provide a cardioplegia delivery system that allows the potassium concentration in the cardioplegia mixture to be varied over a desired range without varying the dilution of the cardioplegia mixture or the concentration of other ingredients. Alternatively, it is desired to permit variation of the potassium and/or other concentrations, while independently varying the degree of dilution and the total flow of cardioplegia mixture to the patient's heart. Varying the potassium concentration in the cardioplegia mixture would allow the perfusionist to minimize the total amount of potassium added to the patient's blood during an operation. It would also allow a high initial potassium concentration to rapidly arrest the heart followed by a lower concentration to maintain arrest. It is further desired to allow for adjustment of the amount of potassium added to the blood to compensate for the increase in the patient's serum potassium level throughout the course of the operation. It is also desired to increase the cardioplegia potassium concentration in the event of a reoccurrence of heart activity during surgery.
U.S. Pat. No. 5,385,540 to Abbott et al., appears to disclose cardioplegia systems which include two crystalloid solutions which can be administered alternatively, allowing two concentrations of cardioplegia to be administered, but not permitting continuous variation of the cardioplegia solution concentration. U.S. Pat. No. 5,385,540 also appears to disclose a system in which cardioplegia crystalloid solution is injected into a crystalloid cardioplegia mixture downstream of a cardioplegia pump. Such a system permits control of the concentration of crystalloid in the cardioplegia mixture but also would vary the dilution of the cardioplegia mixture by the crystalloid solution.
U.S. Pat. No. 5,322,500 to Johnson, et al., appears to disclose a system utilizing two sources of cardioplegia solution which are alternatively selectable.
U.S. Pat. No. 5,358,481 to Todd et al, appears to disclose a variable concentration cardioplegia system in which a plurality of alternatively selectable crystalloid pump headers are placed in the same peristaltic pump raceway with the diverted arterial blood pump header, thereby permitting incremental adjustment of cardioplegia concentrations and dilution of the cardioplegia mixture. However, the dilution of the cardioplegia mixture would be a direct function of the concentration of the crystalloid solution added by selective operation of the alternatively selectable pump headers.
None of the known prior art systems permit continuous variation of crystalloid solution component concentration, and, in particular, potassium concentration in the cardioplegia mixture while maintaining either a fixed dilution ratio between the crystalloid solution and the diverted arterial blood in the cardioplegia mixture. Further, none of the known prior art systems allow control of the dilution ratio independent from control of the concentration of crystalloid in the mixture.
It is against this background that the significant improvements in the art of cardioplegia mixture delivery and open heart surgery of the present invention occurred.